The present invention relates to a small and inexpensive isolator usable in the ranges of VHF, UHF and microwaves.
Isolators are widely used as indispensable parts for microwave apparatuses in wide ranges of microwave applications for the purposes of protecting transistors at high power, interstage matching, removing unnecessary radiations, etc. Recently, because of dramatic miniaturization of other microwave elements, the isolators have come to occupy considerably large space relative to other elements in overall microwave apparatuses. For instance, there are some microwave apparatuses, several tens of percent of whose space is occupied by isolators. Further, a considerable percentage of the costs of the overall microwave apparatuses is attributed to the isolators. Accordingly, demands are increasing for the miniaturization and cost reduction of the isolators.
In general, various types of isolators are already known as shown in FIG. 1 [See, for instance, Konishi et al., "Recent Microwave Circuit Technology Using Ferrite," Denshi Tsushin Gakkai [Electronic Communications Association] pp. 70-104, 1969]. The same reference numerals are assigned to the corresponding parts in all of FIGS. 1 [a]-[f]. Specifically, FIG. 1 [a] shows an isolator utilizing a Faraday effect in a circular waveguide 3a. FIG. 1 [b] shows an isolator having a rectangular waveguide 3 in which the displacement of an electric field is utilized. FIG. 1 [c] shows an isolator having a ferrite slab 1 whose edge guide mode is utilized. FIG. 1 [d] shows an isolator comprising a usual junction circulator 11, one terminal of which is connected with a dummy load 2a. FIG. 1 [e] shows an isolator comprising ferrite members 1 at positions of a circularly polarized wave in a rectangular waveguide 3 for absorbing it by resonance. FIG. 1 [f] shows an isolator comprising a microstrip line for generating a circularly polarized wave for resonance absorption.
In the first four isolators shown in FIGS. 1 [a]-[d], an absorption element 2 or a dummy load 2a is provided for absorbing the energy of a microwave propagating backwardly. On the other hand, in the case of the isolators of resonance absorption type shown in FIGS. 1 [e] and [f], microwave ferrite members 1 themselves act as microwave absorbers. In all of FIGS. 1 [a]-[f], 1 represents a soft ferrite member suitable for a microwave, 2 a microwave absorber, 2a a dummy load, 3 a rectangular waveguide, 3a a circular waveguide, 4 a central conductor of a microstrip line, 5 a ground conductor of a microstrip line, 6 a dielectric member, and H.sub.ext an external magnetic field.
To achieve the miniaturization of an isolator, the smaller the number of parts, the more advantageous. In this sense, the resonance absorption-type isolator, which does not need a microwave absorber separately, appears to be more suitable. However, this type of an isolator is not widely used at present. The reason therefor is not clear, but it may be considered that a means for exciting a circularly polarized wave for resonance absorption is complicated, meaning that the number of parts are not necessarily reduced. Another reason is that since it positively employs a non-linear phenomenon like resonance, the harmonic generation of high-frequency waves undesirable to the microwave apparatuses is inevitable.